1. Field of the Invention
The present invention relates to a self-drilling screw including a head, a thread section adjoining the head, and a drilling section adjoining the thread section and formed of first and second parts arranged one after another, with the drilling section having two, diametrically opposite, discharge grooves extending parallel to a longitudinal axis of the screw at least along a portion of a longitudinal extent of the drilling section, and first and second cutting edges, and with a radial extension of the first and second cutting edges being symmetrical with respect to the longitudinal axis of the screw in the first, in a drilling direction, part of the drilling section, with at least a portion of the second part of the drilling section, which is located between the first part and the thread section, having in a region of the second discharge groove, a radial extension corresponding, at most, to a half of a root diameter of the thread section.
2. Description of the Prior Art
Attachment elements, which are used for securing, e.g., a thin-wall metal constructional part to a constructional component formed as a metallic support, can be formed, e.g., as a self-drilling screw of a type described in European Publication EP-0 535 193. This self-drilling screw has a drilling section which forms a hole in the constructional part and in the metallic support, into which a thread section of the self-drilling screw can be screwed in.
The self-drilling screw has, beside the drilling and thread section, a head which adjoins the thread section at the side of the thread section opposite the drilling section, and which is provided, e.g., with drive surfaces engageable with a rotary tool. The drilling section has two, diametrically opposite, discharge grooves and two diametrically opposite, cutting edges, both extending substantially parallel to the longitudinal axis of the self-drilling screw. Both the discharge grooves and the cutting edges extend at least along a portion of the longitudinal extent of the drilling section. The discharge grooves and the cutting edges extend from the free end of the drilling section. The drilling section is formed of two, arranged one after another, parts, with the cutting edges being arranged, in the first, in the drilling direction, part symmetrically with respect to the longitudinal axis of the self-drilling screw, and with the second part of the drilling section having a diameter which is smaller than the root diameter of the thread section.
Because of the smaller diameter of the second part of the drilling section, no adequate side guidance is provided when the second part of the drilling section of the self-drilling screw passes through the holes formed in the constructional part and its support. This results in an inadequate centering of the following thread section because of, e.g., an eccentrical alignment of the self-drilling screw with respect to the holes, or can result in an inclination of the self-drilling screw relative to the outer surface of the constructional part. This can cause a radial expansion of the holes or damage of thread flanks during driving-in of the thread section. This adversely affects the holding value of the self-drilling screw in the construction part and its support.
Accordingly, an object of the present invention is to provide a self-drilling screw which would insure rapid setting of the self-drilling screw without a large expenditure of force, while providing for adequate guidance and centering of the drilling section.